The exposure of biological cells to an electric field can change the morphology and the function of the cell. The nature and extent of these changes depends foremost on the duration of the exposure and the strength of the applied electric field. The predominant primary response is the charging of cell envelope and subcellular membranes. When a potential difference across the membrane above a certain threshold is eventually reached, membrane lipids become perturbed and as the perturbation is sustained, pores will start to open, enabling molecular transport. Long pulses, i.e. with durations of microseconds or milliseconds, are frequently used as a tool to introduce large molecules such as drugs and DNA into the cell in this manner. With the accumulation of ions along the cell envelope, the potential difference across the membrane increases and the electric field inside the cell is reduced simultaneously.